Back-lit displays are used in a wide variety of computers and consumer devices such as TVs. For example, a back-lit liquid crystal display (LCD) is an array of pixels in which each pixel acts as a shutter that either passes or blocks light from a light source that is located behind the pixel. Color displays are implemented by equipping the pixels with color filters such that each pixel transmits or blocks light of a particular color. The intensity of the light from each pixel is set by the time the pixel is in the transmissive state.
The display is typically illuminated by a white light source that provides a uniform intensity of light across the back surface of the display. Illumination sources based on fluorescent lights are particularly attractive because of their high light output per watt-hour of power consumed. However, such sources require high driving voltages which makes them less attractive for battery operated devices. In addition, the thickness of the light source determines the minimum thickness of the display, and hence, light sources based on fluorescent tubes that are placed behind the LCD display are often too thick for many handheld device applications.
Thinner light sources can be constructed using a light guide consisting of a sheet of clear plastic that is illuminated at one or more edges by a light source. Light entering the sheet is internally reflected, and hence, trapped, until it is scattered by one of the scattering centers that are incorporated in the sheet. A portion of the scattered light then exits the sheet through the top surface of the sheet. If the scattering centers are properly placed in the sheet, the top surface provides a uniformly illuminated two-dimensional light source that can be used to illuminate a back-lit display.
Light guides such as those discussed above are also used to illuminate keypads in handheld devices. The keys can be implemented with a transparency that indicates the key positions or the keys can be implemented with thin film buttons that are mounted on the illuminated surface such that the areas between the keys are illuminated when the keypad is active. Such keypads are utilized in devices such as cellular telephones.
In LED-based displays, the light guides are illuminated by LEDs that are placed on one or more edges of the light guide. To provide a light source for a color LCD display, LEDs that emit light at three different wavelengths are used. Since the LEDs are point sources that emit light in relatively narrow spectral bands, a light mixing zone must be provided at the edge of the light guide and separated from the region to illuminate the display by a distance sufficient to assure that the light trapped under the region to be illuminated is uniform in intensity and color.
There is a trend in handheld devices toward ever-thinner configurations. Since the thickness of the light guide represents a significant portion of the thickness of many devices, the thickness of the light pipes has been reduced to the point that it is difficult to provide good coupling between the LEDs and the edge of the light guide. Light guides with a thickness of 0.4 mm are utilized in some display devices and a need for still thinner light guides exists. The thickness of the light guide is now approaching the size of the smallest LED packages, and hence, it is difficult to provide efficient coupling and mixing of the light from a number of LEDs into the edge of the light pipe.